Online casinos, also known as virtual casinos or Internet casinos, are online versions of traditional ("brick and mortar") casinos. Online casinos enable gamblers to play and wager on casino games through the Internet.
Online casinos generally offer odds and payback percentages that are comparable to land-based casinos. Some online casinos claim higher payback percentages for slot machine games, and some publish payout percentage audits on their websites. Assuming that the online casino is using an appropriately programmed random number generator, table games like blackjack have an established house edge. The payout percentage for these games are established by the rules of the game.

There are a number of types of online casinos at which you can play. Many of them are Java-based and load directly in your browser. Some of them use Flash or Shockwave, which also runs in your browser but may require the Flash (or Shockwave) plug-ins available from Macromedia.
Both of these types are usually referred to as no-download casinos, even though you have to download graphics and sounds every time you play. The biggest advantage of a no-download casino is that it can be played on most computers and operating systems, and many of these will also work on WebTV.